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#include <iostream>
#include <utility>
 
class test
{
private:
	test(int) { }
public:
	test foo() { return *this; }
 
	static const char *name() { return "test"; }
};
 
int main()
{
	std::cout << decltype(test().foo())::name() << std::endl;               // 1
	std::cout << decltype(std::declval<test>().foo())::name() << std::endl; // 2
}
 

prog.cpp: In function ‘int main()’:
prog.cpp:16:29: error: no matching function for call to ‘test::test()’
  std::cout << decltype(test().foo())::name() << std::endl;               // 1
                             ^
prog.cpp:16:29: note: candidates are:
prog.cpp:7:2: note: test::test(int)
  test(int) { }
  ^
prog.cpp:7:2: note:   candidate expects 1 argument, 0 provided
prog.cpp:4:7: note: constexpr test::test(const test&)
 class test
       ^
prog.cpp:4:7: note:   candidate expects 1 argument, 0 provided
prog.cpp:4:7: note: constexpr test::test(test&&)
prog.cpp:4:7: note:   candidate expects 1 argument, 0 provided
prog.cpp:16:29: error: no matching function for call to ‘test::test()’
  std::cout << decltype(test().foo())::name() << std::endl;               // 1
                             ^
prog.cpp:16:29: note: candidates are:
prog.cpp:7:2: note: test::test(int)
  test(int) { }
  ^
prog.cpp:7:2: note:   candidate expects 1 argument, 0 provided
prog.cpp:4:7: note: constexpr test::test(const test&)
 class test
       ^
prog.cpp:4:7: note:   candidate expects 1 argument, 0 provided
prog.cpp:4:7: note: constexpr test::test(test&&)
prog.cpp:4:7: note:   candidate expects 1 argument, 0 provided
prog.cpp:16:15: error: decltype evaluates to ‘<type error>’, which is not a class or enumeration type
  std::cout << decltype(test().foo())::name() << std::endl;               // 1
               ^

Standard output is empty